Over the last several decades, well logging methods have become very sophisticated. Many new procedures, such as Nuclear Magnetic Resonance (NMR), have been used in the testing of well strata. NMR methods have proven useful in determining whether a particular well will be productive, since producible fluids are easily distinguishable by their slow NMR relaxation times.
Estimating permeability of sedimentary formations is one of the most important factors in distinguishing economic from uneconomic reservoirs. Generally, however, the estimation of permeability from log data has been only partially successful. In the past, diversified core data did not clearly correlate with estimated results. It was sometimes difficult to form a straight-line semi-logarithmic plot of permeability as a function of porosity through the data points. Such plots often showed a significant amount of residuals above and below the line.
The present invention introduces an improved method of estimating the permeability of sedimentary rock formations.
The current invention uses nuclear magnetic resonance measurements of the longitudinal or transverse relaxation time constant on the same plug samples used to determine permeability. The equation governing the surface relaxation is combined with a permeability equation using the lambda parameter suggested by Johnson, D. L., Koplik, J. and Schwartz, L. M., "New Pore-size Parameter Characterizing Transport in Porous Media", Physical Review Letters, (1986), 57, 2564-2567. A new, alternate permeability expression is thus provided, having a 0.92 correlation coefficient. The NMR form of the k-Lambda equation appears to be useful, however, only for water-bearing zones, as the values were susceptible to change when hydrocarbon replaces water in the pores.
In an alternate embodiment of the method, the invention then uses a mineral form of k-Lambda. The surface-to-pore volume ratio is expanded into terms relating to the porosity, and to the minerals present. Clay minerals, which contribute most to the surface area of reservoir sands and shaly sands, tend to cluster into fairly unique values of specific surface area. Thus, it was expected that the specific surface area of sediments could be constructed as a linear combination of the mass fractions of the minerals present. The new k-Lambda estimate has a remarkable correlation coefficient of 0.99.
One of the important advantages of the inventive method is that the k-Lambda technique can be activated using log data. The NMR form of the k-Lambda method uses total porosity and magnetic resonance measurements (relaxation times, T.sub.1 or T.sub.2) data. The mineral form of k-Lambda utilizes total porosity and estimates of the mineral fractions.
Moreover, seismic information can also be incorporated in the inventive method due to well-known correlations among velocity, porosity and clay content, as discussed, for example, in "Modeling the Relationships Between Sonic Velocity, Porosity, Permeability, and Shaliness in Sand, Shale, and Shaley Sand", by D. Marion, A. Nur, and F. Alabert, SPWLA Thirtieth Annual Logging Symposium,
Jun. 11-14, 1989.
Another important advantage of the method of this invention is that the k-Lambda technique is a robust means of estimating permeability.